Prostatic fluid which can flow from the penis in the form of a mini-orgasm. Unlike a full ejaculation which produces cum, cream, or semen, a prostate-stimulated discharge is not explosive. Milk can be white or somewhat transparent and is much thinner than real cum.
Another difference from a full-fledged ejaculation is that the penis usually becomes even more excited after discharging milk and since the urethra has been primed, the ensuing orgasm is often felt to be much hotter.

I massaged his prostate with a dildo in his asshole and milk streamed out of his dick.
I fucked that boy so good, ramming his prostate with my cock, that streams of milk flowed out of his stiffy.
Milk before cream.